METHODS OF APPLICATION

14.1 METHODS OF APPLICATION

Metal coatings are applied by dipping, electroplating, spraying, cementation, and diffusion. The selection of a coating process for a specifi c application depends on several factors, including the corrosion resistance that is required, the anticipated lifetime of the coated material, the number of parts being produced, the produc- tion rate that is required, and environmental considerations.

Hot dipping is carried out by immersing the metal on which the coating is to

be applied, usually steel, in a bath of the molten metal that is to constitute the coating, most commonly zinc, but also aluminum and aluminum – zinc alloys. Hot dipping can be either a continuous process, as in galvanizing steel sheet, or a batch process — for example, galvanizing fabricated parts, nuts, bolts, and fasteners [1] .

In electroplating , the substrate, or base, metal is made the cathode in an aqueous electrolyte from which the coating is deposited. Although the primary purpose of electroplating coatings is to achieve corrosion resistance, these coat- ings can also be decorative, with a metallic luster after polishing [2] . A wide of variety of coatings can be applied by electroplating — for example, zinc, cadmium, chromium, copper, gold, nickel, tin, and silver, as well as alloys, such as tin – zinc,

Corrosion and Corrosion Control , by R. Winston Revie and Herbert H. Uhlig Copyright © 2008 John Wiley & Sons, Inc.

270 METALLIC COATINGS

zinc – nickel, brass, bronze, gold alloys, and nickel alloys [2] . Electrogalvanizing is the electroplating of zinc on either iron or steel. Electroplated zinc - coated sheet is widely used for exposed automobile body panels because of its uniform coating thickness and surface characteristics compared to hot - dip zinc - coated. Coatings range in thickness from 4 to 14

μ m [3] .

Coatings are also produced by electroless plating — that is, by chemical reduc- tion of metal – salt solutions, with the precipitated metal forming an adherent overlay on the base metal. Nickel coatings of this kind are called electroless nickel plate.

In thermal spraying of metal coatings, a gun is used that simultaneously melts and propels small droplets of metal onto the surface to be coated. There are several types of thermal spraying, with the three main variables in each type being the temperature of the fl ame, the velocity of the particles that are sprayed onto the substrate to form the coating, and the nature of the material that is to form the coating (i.e., powder, rod, wire, or liquid) [4, 5] . The material that is to form the coating is called the “ feedstock. ” In all cases, the feedstock is rapidly heated and propelled toward the substrate where, on impact, it consolidates forming an adher- ent coating. In fl ame - powder spraying, powder feedstock is melted and carried by the fl ame onto the workpiece. In fl ame - wire spraying, the fl ame melts the wire, and

a stream of air propels the molten material onto the workpiece. In plasma spraying,

a plasma at a temperature of about 12,000 ° C is formed, and the plasma stream carries the powder feedstock to the workpiece [4] . Thermally sprayed coatings tend to be porous, although porosity can be controlled by optimizing the process variables [4] . These coatings can be made adherent and of almost any desired thickness, and they can be applied on already fabricated structures. Sometimes, pores are fi lled with a thermoplastic resin in order to increase corrosion protection.

Cementation consists of tumbling the work in a mixture of metal powder and

a fl ux at elevated temperatures, allowing the metal to diffuse into the base metal. Aluminum and zinc coatings can be prepared in this way. Diffusion coatings of chromium, nickel, titanium, aluminum, and so on, can also be prepared by immers- ing metal parts, under an inert atmosphere, in a bath of molten calcium containing some of the coating metal in solution [6] .

Coatings are also sometimes produced by gas - phase reaction . For example, CrCl 2 , when volatilized and passed over steel at about 1000 ° C (1800 ° F), results in formation of a chromium – iron alloy surface containing up to 30% Cr in accord with the reaction

3 3 CrCl 2 + Fe → FeCl 3 + Cr alloyed with Fe ( )

Similar surface alloys of silicon – iron containing up to 19% Si can be prepared by reaction of iron with SiCl 4 at 800 – 900 ° C (1475 – 1650 ° F). Ion implantation is a process of producing thin surface alloy coatings by bombarding the metal with ions in vacuum. Such coatings of, for example, Ti, B,

CL ASSIFIC ATION OF COATINGS

Cr, or Y have specialized applications for wear and high - temperature oxidation resistance [7, 8] .